1. Field
This disclosure relates generally to passive audio ear filters that can attenuate certain sound frequencies and allow other frequencies to pass through unchanged.
2. Description of the Related Art
Exposure to sound at certain sound pressure levels and sound frequencies can, over time, cause hearing loss.
Humans' perception to sound varies with both frequency and sound pressure level (SPL). For example, humans do not perceive low and high frequency sounds as well as they perceive midrange frequencies sounds (e.g., 500 Hz to 6,000 Hz). Further, human hearing is more responsive to sound at high frequencies compared to low frequencies. FIG. 1 illustrates equal loudness contours defined in ISO (International Standards Organization) Standard 226(2003). The X axis represents sound frequency measured in Hertz (Hz) and the Y axis represents sound pressure level measured in decibels (dB) relative to a pressure level of 2×10−5 Pascal. The unit of measurement for loudness levels is the phon, and is arrived at by reference to equal-loudness contours. FIG. 1 shows equal loudness contours for loudness levels of 20, 40, 60, 80, and 100 phon. Each equal-loudness contour defines the sound pressure level, over the frequency spectrum, for which a listener perceives a constant loudness when presented with pure steady tones. FIG. 1 also shows the hearing threshold level.
Hearing protection that attenuates sound equally at all frequencies, or otherwise without regard to the variation of hearing sensitivity with frequency, may attenuate potentially damaging sounds at the expense of pleasurable sounds. For example, an ear filter providing uniform attenuation of 20 dB would reduce loudness by about 20 phon at 1 kHz and 40 phon at 20 Hz. Thus the relative loudness of low frequency sounds would be substantially reduced relative to the loudness of higher frequency sounds. However, with attention to the hearing response curves, it is possible to design ear filters that attenuate damaging sound levels and maintain, or even enhance, desired sounds.
There are many situations where people desire protection from sounds at certain frequencies, while allowing sounds at other frequencies to reach their ears. For example, at a concert, concert goers might want to enjoy the music, but also be protected from the mid-range and high levels of sound frequencies that cause damage to a person's hearing. On an airplane, passengers might wish to block out the roar of the engine, but not conversation. At a sports event, fans might desire to hear the action of the game, but receive protection from the roar of the crowd. While sleeping, people might want protection from all auditory disturbances. These are just a few common examples where people wish to hear some, but not all, of the sound frequencies in their environment.